Implement hanging slurs

[craigsapp]

Hanging slurs should be implemented:

**kern
=1
2c
=2
(2e
=3
2f
=
*-

These will be problematic for cases of potentially incorrect hanging slurs, which can be shown with the slurcheck tool:

One possibility would be to disable hanging ties if the slurcheck filter is active so that there is not a mess of incorrect hanging ties messing up the notation during data entry.